I have lived in Hamburg for three years and in all that time I have only managed to go there once or twice, the one and only reason being that it is on so terribly early on a Sunday morning. The Fischmarkt starts at about 5 am, maybe even a bit earlier and it finishes around ten, but is is recommended to go there as early as possible, otherwise the best stuff might be gone. It is definitely worth buying there if you consume large quantities of fish / vegetables / flowers. If you are a single household though, it might not be worth it, unless you consume loads of fruit / fish / flowers.
For about 20 Euros (or was it 20 DMark? Cant remember, its been a while ago) you can get a huge wicker basket full of fruit, like pineapples, kiwis, oranges, apples, bananas..., simiolar bargains for fish, especially smoked eel (defo worth trying, my English Bf was sceptical at the beginning, but now he loves it). It is good fun, because the salespeople "Marktschreier" are good fun to watch as they scream at the crowds to sell their products.
As it is just a few meters from "Reeperbahn", lots of people walk / stumble there after a night out.
Have a good time in Hamburg.